This invention concerns a method of forming a metal wheel, particularly a one-piece metal wheel suitable for use on automotive and truck vehicles.
The vast majority of automotive vehicle wheels are presently made of steel, and conventionally are made by cutting and welding a piece of steel to form a circular tube and then roll-forming this tube to the configuration of the wheel rim. A separate piece of steel is formed to the shape of the disc or center of the wheel and then welded to the rim. Such steel wheels are strong and relatively inexpensive to make.
However, in recent years there has been a determined effort to reduce the weight of vehicles, particularly automobiles. To this end, the use of aluminum in place of steel to make vehicle wheels is very attractive because of the much lower weight per unit volume of aluminum as compared to steel.
It is known to make two-piece wheels of aluminum, first forming the rim and then welding it to the disc or center of the wheel; see U.S. Pat. No. 4,185,370. It is also known to extrude the rim portion of a two-piece wheel; (see U.S. Pat. Nos. 2,034,360 and 3,172,787). However, the welding operation can cause problems: not only must aluminum be welded very carefully, but the metallurgical, particularly the strength, characteristics of the metal in the weld are generally inferior to that of aluminum which has not been subjected to the high heat inherent in the welding operation. In other words, the metal in the weld is generally not as strong as the metal in the other parts of the wheel, leading to a point of potential weakness in the wheel.
It is also known to form one-piece aluminum wheels by casting the complete wheel and also by forging the wheel to shape (see British Pat. Nos. 971,258 and 971,259), or by forging and spinning portions of the wheel to shape (see U.S. Pat. Nos. 4,048,828 and 4,055,068). However, the main problem with casting or forging one-piece aluminum wheels is that, in order to achieve adequately strong wheels, it is necessary to use relatively thick sections of aluminum (as compared to a steel wheel, for example) and thus the weight advantage of using aluminum is more or less totally lost.
The method of the present invention permits the fabrication of a one-piece aluminum wheel using a minimum amount of metal. In other words, portions of the wheel which must be stronger can be formed to a thicker section and portions which require less strength can be formed to a thinner section. Furthermore, the wheel produced according to the method of this invention is free of the zones of relative weakness associated with hot welded joints.